Wheel loss from vehicles such as trucks and cars is a problem that can lead to serious accidents and fatalities, and is a problem that can be time consuming and expensive to address. A Transport Research Laboratory (TRL) report on wheel loss on commercial vehicles, published for the Department for Transport (Dft), estimated that the typical annual frequency of wheel fixing problems is as follows for the UK alone.                Between 7,500 and 11,000 wheel fixing defects.        Between 150 and 400 wheel detachments.        Between 50 and 134 resulting in damage only accidents.        Between 10 and 27 resulting in injury accidents.        Between 3 and 7 fatal accidents.        
Wheel loss is a serious problem, and there are several solutions that attempt to reduce the likelihood of wheel loss.
Wheels are typically affixed to a vehicle axle using wheel nuts. The UK Department for Transport recommends that any nuts, studs or bolts purchased for fixing wheels comply with British Standard AU 50: Part 2 Section 7a: 1995 for commercial vehicles, or with British Standard AU 50: Part 2 Section 8a: 1985 for cars, to ensure they are of high quality. By specifying a minimum quality for nuts, studs and bolts, this reduces the likelihood that these parts will fail through shear or another mechanism, but it cannot entirely solve the problem. Furthermore, shear isn't the only cause of wheel loss; vibration can cause wheel nuts to loosen, allowing the wheel to become detached.
A number of safety devices are available that are designed to help keep wheel nuts tight or visually indicate if nuts are becoming loose. For example, a wheel nut locking device is available that prevents wheel loss caused by loosening of the wheel not owing to vibration. The device uses a modified wheel stud and a spring-loaded counter-threaded locking cap that covers the wheel nut, holding it in place. The device does not interfere with the original nut, and so it maintains maximum clamp force. In the event of the wheel nut starting to loosen, the locking cap tightens against the wheel nut, locking it into place, to ensure that the wheel does not become detached from the vehicle. This device may be used on trucks, buses and coaches, and the device has also been developed to ensure bolt security in the rail industry.
Another type of device replaces an existing wheel nut with a heavy-duty locking nut, and aims to stop the possibility of losing wheels from vehicles. The locking nut is split into three sections; a nut, a hexagon-flanged washer and a flat faced cup washer. The top two sections have interlocking cams. When subjected to vibration, the interlocking cams attempt to rise against each other. As the angle of the cam is greater than the pitch angle of the thread on the stud, a wedging action takes place that causes the heavy duty locking nut to maintain the clamping-force and to lock, thereby maintaining the wheel secure on the axle.
A different approach to the problem of vibration loosening is to provide an indication to show when a nut has started to become loose. A plastic indicator is placed between the nut and the wheel, and is visible on an outer rim of the wheel. Each indicator is aligned with the radius of the wheel when the nut is tightened. If a wheel nut starts to loosen, the plastic indicator will be free to move and will no longer be aligned with the radius of the wheel. A visual inspection is required to determine whether or not a wheel nut has loosened.
The existing solutions described above are either nut locking systems or nut movement indication systems. However, wheel loosening cannot be observed within the cabin of the vehicle and hence an occurrence at the beginning of a journey could go undetected on the way to failure. Furthermore, most systems are easily attached therefore can easily become detached.